Work will be conducted in the following areas: 1. Platelet membrane markers; immunologic and immunochemical characterization: The platelet membrane receptor for drug-induced platelet antibodies and the molecular basis for interaction of drug, antibody, and the receptor will be characterized, using radioactively-labeled drug and 125I-labeled highly purified antibodies. 2. Platelet heterogeneity and kinetics of circulating platelets: Kinetic studies of human platelet subpopulations and investigation of the biochemical and structural properties of such populations will be continued in an attempt to further characterize the biological significance of platelet heterogeneity. 3. Immune thrombocytopenias: Serologic and clinical studies of patients with acute "idiopathic" thrombocytopenic purpura and members of their families will be conducted to characterize the natural history of acute ITP and factors involved in its pathogenesis. The usefulness of newly devised assay for measurement of platelet-associated IgG will be explored with emphasis on detection on autoantibodies in "idiopathic" thrombocytopenic purpura. 4. Short-term platelet preservation: The relationship between contaminating white blood cells and the tolerance of platelet concentrates to storage will be further characterized. Variability in the tolerance to storage of platelets from individual donors related to factors peculiar to the donors themselves will be examined with emphasis on the intrinsic metabolism of these platelets in vitro. 5. Compatibility testing for platelets: Usefulness of a newly developed assay for platelet-associated IgG in predicting effectiveness of transfused platelets will be evaluated.